The present invention relates to an optical pickup device, particularly relates to an optical pickup device capable of recording and reproducing of information for both optical information recording media of a magneto-optical disc and a write-once-read-many type optical disc (referred to as WORM type optical disc hereafter).
There is widely known read-only optical disc memories (such as the ones for a laser disc and a compact disc) which read and reproduce information signals recorded beforehand on a disc with laser light. In recent years, attention being paid to the largeness of the information recording capacity of an optical disc, attempts is made to bring it into practical use as an information recording medium, for example, a computer memory medium and the like.
As already invented memories utilizing optical discs as information recording media, there is known a WORM type optical disc memory that, in use, permits additional recording of information but does not permit erasing nor rewriting of the information, and a magneto-optical disc memory capable of recording, reproducing and erasing of information by using the magneto-optical effect.
As for a WORM type optical disc, information is recorded as pit patterns on a disc surface as on a conventional read-only optical disc. At the time of a reading operation (pickup), information is read in such a way that a spot of laser light is applied to the pit patterns, the spot light reflected from the disc surface is detected as, for example, a variation of the reflectance of the reflected light, namely, a variation of an amount of the reflected light (light and darkness).
In a WORM type optical disc, what is called, a write-once optical disc, the recording on the same part of the disc is carried out only once but the reading is carried out repeatedly. Recording on a WORM type optical disc is performed by alteration of the reflectance such as forming of pits on the recording surface of the disc or transforming of the crystal state of the surface with the laser light for recording, and reproducing of information is performed by using a difference of the amount of the reflected luminous quantities of laser light for reproducing.
While, in a magneto-optical disc, information is recorded as a variation of the state of magnetization in such a way that a recording position where magnetic field is applied to a position at which the laser light is applied, while carrying the information to be recorded with the laser light for recording. In reproducing, information is reproduced by irradiation of the laser light for reproducing and detection of a variation of the state of polarization in the reflected light.
And a magneto-optical disc is so arranged as to make it possible to recording, reproducing, and erasing of information by using, as a recording medium, a disc with the surface where a magnetic thin film having the magneto-optical effect is formed. To be brief about the principles of recording and erasing of information concerning this magneto-optical disc, firstly the whole surface of a magnetic thin film is first magnetized in the direction perpendicular to the disc surface prior to the information recording, at the time of recording, secondly applying the laser light to the recording position while the weak magnetic field is kept applied in the opposite direction to thereby reverse the state of the magnetization there by a heat of the laser light, consequently information is recorded. At the time of erasing, laser light is applied to the erasing position where the magnetic field is kept applied in the same direction as the magnetization direction magnetized before hand to thereby back the magnetization direction there to the original direction and thus erasing is done.
Information is read by converging laser light on the disc surface making use of the phenomenon (called the magneto-optical effect) that the plane of polarization of the reflected light rotates differently depending on the magnetization direction.
An optical pickup device for a magneto-optical discs can not serve as a combined use with one for WORM type optical disc because the fluctuation in the power of laser light emitted from a semiconductor laser has seriously undesirable influence on a reproducing operation of a WORM type optical disc.
Therefore the conventional optical pickup device can not serve as a combined use as both magneto-optical disc and WORM type optical disc, and the optical pickup device intended for a combined use as both magneto-optical type optical disc and WORM type optical disc is difficult to be realized.
If a conventional optical-pickup device is used as a reproducing device intended for a combined use as both magneto-optical type optical disc and WORM type optical disc, the fluctuation in the power of the reflected light, which is not a problem when reproducing in the magneto-optical way, will drastically increase when reproducing in the WORM way. The fluctuation in the intensity of the reflected light, since it is generated when a spot light is focused on a disc, increases the power to eventually cause deterioration of the medium more at actual reproducing time when focus servomechanism continues to focus every moment than at the time of setting of the reproducing power. Additionally the fluctuation in the luminous quantity causes noise and have undesirable influence on a reproduced signal.
If a single and the same optical pickup ensures recording and reproducing for each optical information recording medium of the magneto-optical disc and the WORM type optical disc, that will be extremely convenient.
In order to realize the optical pickup usable in common to the magneto-optical disc and the WORM type optical disc, the following problems must be settled.
That is, what must be settled is the problem of the output fluctuation due to return light in a semiconductor laser, which is the most suitable for a light source for an optical pickup.
The output of the semiconductor laser, if return light strikes it, fluctuates. This output fluctuation owing to return light is remarkable when the output is small but so decreases its power as to be negligible when the output become large enough. In other words, the output fluctuation becomes serious only when the semiconductor laser is so driven that its output power is low.